Replication of bovine papillomavirus vectors in murine cells

Replication and partitioning of papillomavirus genomes

Papillomaviruses establish persistent infection in the dividing, basal epithelial cells of the host. The viral genome is maintained as a circular, double-stranded DNA, extrachromosomal element within these cells. Viral genome amplification occurs only when the epithelial cells differentiate and viral particles are shed in squames that are sloughed from the surface of the epithelium. There are three modes of replication in the papillomavirus life cycle. Upon entry, in the establishment phase, the viral genome is amplified to a low copy number. In the second maintenance phase, the genome replicates in dividing cells at a constant copy number, in synchrony with the cellular DNA. And finally, in the vegetative or productive phase, the viral DNA is amplified to a high copy number in differentiated cells and is destined to be packaged in viral capsids. This review discusses the cis elements and protein factors required for each stage of papillomavirus replication.

Episomal maintenance of plasmids with hybrid origins in mouse cells

Bovine papillomavirus type 1 (BPV1), Epstein-Barr virus (EBV), and human herpesvirus 8 genomes are stably maintained as episomes in dividing host cells during latent infection. The mitotic segregation/partitioning function of these episomes is dependent on single viral protein with specific DNA-binding activity and its multimeric binding sites in the viral genome. In this study we show that, in the presence of all essential viral trans factors, the segregation/partitioning elements from both BPV1 and EBV can provide the stable maintenance function to the mouse polyomavirus (PyV) core origin plasmids but fail to do so in the case of complete PyV origin. Our study is the first which follows BPV1 E2- and minichromosome maintenance element (MME)-dependent stable maintenance function with heterologous replication origins. In mouse fibroblast cell lines expressing PyV large T antigen (LT) and either BPV1 E2 or EBV EBNA1, the long-term episomal replication of plasmids carrying the PyV minimal origin together with the MME or family of repeats (FR) element can be monitored easily for 1 month under nonselective conditions. Our data demonstrate clearly that the PyV LT-dependent replication function and the segregation/partitioning function of the BPV1 or EBV are compatible in certain, but not all, configurations. The quantitative analysis indicates a loss rate of 6% per cell, doubling in the case of MME-dependent plasmids, and 13% in the case of FR-dependent plasmids in nonselective conditions. Our data clearly indicate that maintenance functions from different viruses are principally interexchangeable and can provide a segregation/partitioning function to different heterologous origins in a variety of cells.

Episomal vectors for gene expression in mammalian cells

An important reason for preferring mammalian cells for heterologous gene expression is their ability to make authentic proteins containing post-translational modifications similar to those of the native protein. The development of expression systems for mammalian cells has been ongoing for several years, resulting in a wide variety of effective expression vectors. The aim of this review is to highlight episomal expression vectors. Such episomal plasmids are usually based on sequences from DNA viruses, such as BK virus, bovine papilloma virus 1 and Epstein-Barr virus. In this review we will mainly focus on the improvements made towards the usefulness of these systems for gene expression studies and gene therapy.

Development of human papillomavirus plasmids capable of episomal replication in human cell lines

Gene delivery into human cells has been facilitated by the development of viral vector systems. These vectors have shown great potential for the efficient delivery of therapeutic genes into human cells. A problem with many of the existing systems, however, is the integration of these vectors into the chromosome which affects the length of gene expression and may promote oncogenic transformation. In an effort to develop viral vectors that can replicate extrachromosomally in human cells, we have generated human papillomavirus (HPV) plasmids containing all the elements required for replication on a single DNA molecule. HPV plasmids containing the viral E1 and E2 genes (or the E1 gene alone) and an origin of replication were shown to replicate to significant levels in the transfected human cervical carcinoma C-33A cell line. Since approaches towards the possible gene therapy of cystic fibrosis (CF) are currently under intensive investigation, we have also tested short-term replication of HPV plasmids in the IB3 cell line derived from a CF patient. Our results demonstrate that HPV plasmids are capable of extrachromosomal replication in these cell lines and may potentially be important vectors for the delivery of therapeutic genes into human cells.

The evolution of small DNA viruses of eukaryotes: past and present considerations

Historically, viral evolution has often been considered from the perspective of the ability of the virus to maintain viral pathogenic fitness by causing disease. A predator-prey model has been successfully applied to explain genetically variable quasi-species of viruses, such as influenza virus and human immunodeficiency virus (HIV), which evolve much faster rates than the host. In contrast, small DNA viruses (polyomaviruses, papillomaviruses, and parvoviruses) are species specific but are stable genetically, and appear to have co-evolved with their host species. Genetic stability is attributable primarily to the ability to establish and maintain a benign persistent state in vivo and not to the host DNA proofreading mechanisms. The persistent state often involves a cell cycle-regulated episomal state and a tight linkage of DNA amplification mechanisms to cellular differentiation. This linkage requires conserved features among viral regulatory proteins, with characteristic host-interactive domains needed to recruit and utilize host machinery, thus imposing mechanistic constrains on possible evolutionary options. Sequence similarities within these domains are seen amongst all small mammalian DNA viruses and most of the parvo-like viruses, including those that span the entire spectrum of evolution of organisms from E. coli to humans that replicate via a rolling circle-like mechanism among the entire spectrum of organisms throughout evolution from E. coli to humans. To achieve benign inapparent viral persistence, small DNA viruses are proposed to circumvent the host acute phase reaction (characterized by minimal inflammation) by mechanisms that are evolutionarily adapted to the immune system and the related cytokine communication networks. A striking example of this is the relationship of hymenoptera to polydnaviruses, in which the crucial to the recognition of self, development, and maintenance of genetic identity of both the host and virus. These observations in aggregate suggest that viral replicons are not recent "escapies" of host replication, but rather provide relentless pressure in driving the evolution of the host through cospeciation.